Mid-frequency AC Sputtering has become a mainstay of thin film sputtering technologies, particularly for the deposition of dielectric or non-conducting film coatings on surfaces such as solar panels, optical glass such as telescope mirrors or rolls of plastic. It is largely replacing RF Sputtering for coating dielectrics because it operates in the kHz rather than MHz range requiring less sophisticated and expensive power sources and is a process that is adaptable to large scale applications.

MF or Mid-frequency AC power supplies cover a wide range of voltage outputs between 300 V to 1200 V - generally in the 25 to 300 kW range - at frequencies between 20 to 70 kHz with 40 kHz used most commonly. It is a process frequently used with Reactive Sputtering where a reactive gas such as oxygen or nitrogen is introduced into the plasma to form oxides or nitrides on the substrate.

Two cathodes are used with an AC current switched back and forth between them which cleans the target surface with each reversal to reduce the charge build up on dielectrics that leads to arcing which can spew droplets into the plasma and prevent uniform thin film growth.

Diagram of the Sputtering Process

Combined with sophisticated arc detection and suppression circuitry, MF or Mid-frequency AC Sputtering offers the advantages of improving process stability and increasing deposition rates as well as overcoming the problem known as the “Disappearing anode” effect. When trying to reactively sputter a dielectric coating with DC sputtering, the anode can become coated with an insulating coating and disappear. This is known as the disappearing anode effect. In the case of AC sputtering, the cathodes act as an anode every half cycle and provide a “clean” anode surface.

Mid-frequency AC Sputtering often employs dual magnetrons to confine the electrons above the target and reduce arcing for process control. These include what are known as either “Balanced” or “Unbalanced” magnetrons that can be arranged side by side, tilted towards each other, or face to face.

Conventional magnetrons are most commonly of the balanced type, where the null point or area where the opposing magnetic fields cancel each other out is usually high in the plasma above the target. When the null point is high above target, there is the least chance of electrons escaping the magnetic field.

Compared with balanced magnetrons, unbalanced magnetrons possess stronger magnets with the null point closer to the target surface, so electrons can escape more easily from the electron trap into the plasma. These electrons undergo ionizing collisions with the inert gasses like Argon farther away from the target surfaces in the plasma and closer to the substrate being coated.

AC Sputtering with unbalanced magnetrons has the great advantage of allowing a bias or charge to be applied to the substrate being coated which effectively forms a secondary plasma in the vicinity of the substrate. As gas ions from this secondary plasma accelerate towards the substrate it produces a much denser plasma around the substrate compared to balanced magnetrons. This results in a much higher ion bombardment of the substrate with unbalanced magnetrons which can be used to control and enhance the properties of the thin film being deposited.

While unbalanced magnetrons can produce coatings which are denser due to the intensified plasma and ion bombardment close to the substrate being coated, the heat load to the substrate will be increased leading to higher substrate temperatures. The energy required to run balanced and unbalanced magnetrons is the same, however the amperage/voltage characteristics change with the unbalanced magnetrons running at higher voltages due to less efficient electron trapping. The primary difference between balanced and unbalanced is the strength of the magnets on the outer pole as compared to the center pole.

Because unbalanced magnetrons create a magnetic field with the null point closer to the target material this results in a more uneven target utilization known as “Race track erosion” which is a round circular depression that forms on the surface of the target material where atoms have been sputtered off into the plasma that has taken on the circular shape of the magnetic field holding the electrons in place. This uneven race track erosion may result in significant expenses as a result of reduced target utilization unless the unbalanced magnetron effect is compensated for and must be considered arriving at the best solution for your specific thin film deposition process.

MF or Mid-frequency AC Sputtering has become the thin film deposition technique of choice for many types of physical vapor deposition processes due to its lower power levels compared to more complicated and expensive power sources like RF Sputtering, improved process stability, increased deposition rates that allow for customizable coatings with the co-sputtering of reactive gases, as well as overcoming the problem known as the “Disappearing anode” effect.